It is well known to produce microelectromechanical systems (MEMSs, standing for Micro Electro Mechanical Systems) comprising a mechanical structure of built-in beam (“cantilever”) type extending from the edge of a planar substrate and which is able to flex in a plane perpendicular to that of the substrate. Such structures have been used, notably, to produce atomic force microscope probes. They operate well in a vacuum or in air, but not in a liquid medium since the vibrations of the beam are then heavily damped by the viscosity of the medium. To solve this problem, it has been necessary to develop MEMSs exhibiting a mechanical structure of beam type flexing in the plane of the substrate or exhibiting a translation motion in this same plane. In the latter case, the beam can be rigid and its translation motion be induced by the flexion—also in the plane—of one or more lateral bracing arms.
A need therefore exists for a microelectromechanical system making it possible to actuate—and to detect—a flexion motion in the plane of a structure of beam type (or, more generally, exhibiting an elongate shape).
A certain amount of research work has proposed that this need be addressed by using actuators of thermal type and piezo-resistive detectors. Such a solution exhibits the advantage of being compatible with the use of a large number of materials contrary, for example, to piezoelectric transduction. Furthermore one and the same structure can serve both as thermal actuator (the heat being produced through the Joule effect) and as piezo-resistive detector.
The article by J. H. Comtois, V. M. Bright and M. W. Phipps “Thermal microactuators for surface-micromachining processes”, Proc. SPIE 2642, Micromachined Devices and Components, 10 (15 Sep. 1995) describes a silicon flexion structure, composed of two parallel arms, linked together by their distal end (opposite to the anchorages) and carrying a resistive track forming a “U”, which makes it possible to pass a current between the two anchorages. This current gives rise to heat through the Joule effect; one of the arms (“cold arm”) is intentionally enlarged so as to promote convective exchanges with the exterior and to decrease the power density in the silicon; thus this arm heats up less than the other (termed the “hot arm”). The more significant thermal expansion of the hot arm then causes flexion of the structure in the plane. This structure is designed for static operation; in any event, the relatively significant mass of the cold arm would penalize any rise in frequency. Furthermore, the resistive track necessarily exhibits a significant length (about twice that of the structure), and therefore a high resistance. Now, in order to obtain efficient power transfer between the transducer and external electronic appliances (power supplies, signal generators, measurement circuits, etc.), the impedance of the transducer ought to be as close as possible to 50Ω (Ohms).
The article by L. A. Beardslee, A. M. Addous, S. Heinrich, F. Josse, I. Dufour and O. Brand “Thermal Excitation and Piezoresistive Detection of Cantilever In-Plane Resonance Modes for Sensing Applications” Journal of Microelectromechanical Systems, Vol. 19, No. 4, August 2010 describes a structure comprising a silicon beam clamped at one end and carrying, near its anchorage and on opposite sides, two resistances obtained by doping. Metallic tracks connect the ends of the resistances and enable an electric current to be injected thereinto. The injection of a current into the resistance on the right causes heating through the Joule effect, which in its turn induces a thermal expansion of the right side of the beam and therefore its flexion towards the left; reciprocally in the case where current is injected into the left resistance. A structure forming a Wheatstone bridge—also obtained by virtue of resistive regions fabricated by doping and interconnected by metallic tracks—allows measurement of the flexion through the piezo-resistive effect. The width of the beam goes from 45 and 90 microns and its length from 200 to 1000 microns; one is therefore dealing with a structure of relatively significant size. A high degree of miniaturization—for example by taking a width of 5 μm—would encounter several problems; in particular, it would be necessary to use very fine and very closely spaced conducting tracks, which would lead to difficulties with insulation and to a high impedance.
More generally, the prior art does not make it possible to obtain transducers of resistive type (thermal actuators and/or piezo-resistive sensors) which are both miniaturizable—this being necessary to allow operation at high frequency (greater than or equal to 1 MHz)—and of low impedance (less than or equal to 100Ω, preferably substantially equal to 50Ω)—thereby allowing efficient electrical power transfer.